German police believe the person or people responsible for a deadly attack on a Berlin Christmas market could still be at large after admitting they might have arrested the wrong man. Holger Münch, head of the federal criminal police, said: “We need to work on the assumption that an armed perpetrator is still on the loose. As a result of this we are on high alert.”

The German chancellor, Angela Merkel, has visited the scene of what she said was an assumed “terrorist attack”. “It would be particularly hard to bear for all of us if it was confirmed that a person committed this crime who asked for protection and asylum in Germany,” she said.

But Münch admitted there is still a “question mark” over whether the attacker was an Islamist. Chief prosecutor Peter Frank said that investigators were assuming that it could have been a terrorist because of the number of people killed and similarities with an attack in Nice in July. But he stressed that nothing was proven

Twelve people were killed in the attack, including six who have been identified as German. A further 48 people were wounded including 18 with “very serious injuries”.

Police said a man found dead inside the truck, identified as a Polish citizen, was not the person who drove it into the market. He was stabbed and shot with a pistol but the weapons have not been found.

A Pakistani man arrested 2km from the scene has denied involvement. “At the moment it is unclear if he really was the driver,” said the chief of Berlin’s police, Klaus Kandt.

Early on Tuesday, police reportedly raided a hangar at the disused Tempelhof airport, part of which is being used to house refugees.

Berlin police said they were investigating if the truck was stolen from a construction site in Poland. The Polish company that owns the truck said its 37-year-old driver, who was transporting steel beams, had been due to take a break in Berlin but had not been heard from since Monday afternoon.

The White House condemned what it said “appears to have been a terrorist attack”. The president-elect, Donald Trump, called it a “horrifying terror attack”, blaming “Isis and other Islamist terrorists [who] continually slaughter Christians in their communities and places of worship”.

Russia’s president, Vladimir Putin, said the attack was “savage in its cynicism”.

The rightwing populist party Alternative for Germany claimed the country’s Christian tradition was under attack. Party leader Frauke Petry said: “The Christmas market was not an accidental target. It is not only an attack on our freedom and our way of life, but on our Christian tradition. Germany is a country which is divided over the immigration question.”

Ukip’s former leader Nigel Farage said “events like these will be the Merkel legacy”.

The Metropolitan police in London is reviewing security at Christmas events in light of the attack.